In their simplest form ceramic capacitors consist of a relatively thin wafer or layer of desired shape and size formed by firing or sintering a ceramic dielectric composition, the layer having electrodes sintered on to the opposite faces thereof. In many cases, however, it is desired to use a capacitor having a plurality of such layers alternating with conductive layers, i.e. electrodes. Alternate electrodes being exposed at the same edge faces of the capacitor and electrically joined there, for example, by a termination electrode.
In a typical known method for producing such ceramic capacitors an electroding paste, sometimes called ink, including a noble metal, such as platinum or palladium, is applied to the top face of a small, usually cast, thin sheet of a suitable ceramic dielectric composition bonded with an heat vaporizable organic binder, the application being made in such manner that the deposit of electroding paste extends to one edge of the sheet only and a clear margin is left around three sides of the deposit. A plurality of the small sheets thus coated with electrode paste are then stacked, successive sheets being rotated about an axis normal to the plane of the sheet, whereby successive electroding paste deposits extend to opposite edges of the stack. The stack of paste-coated sheets is then suitably consolidated and heated to drive off or decompose the organic binders of the ceramic sheet and the electroding paste and to sinter the dielectric composition into a unitary, multilayer body having successive electrodes exposed on opposite ends thereof. The electrodes exposed on each end are then electrically connected with a termination electrode in known manner. The noble metals are used to form the electrodes because their melting points are higher than typical sintering temperatures and their resistance oxidation.
Because of the necessity for using noble metal internal electrodes in the process just described, such ceramic capacitors are expensive. Lower cost metals, such as silver can be used for electrodes if the dielectric compositions can be sintered at low enough temperatures to avoid melting, vaporization, and oxidation of the silver. Moreover, silver, while cheaper than the noble metals, is still very expensive. Accordingly, a method of producing capacitors that does not require the use of noble or very expensive metals is desirable.
The method disclosed in U.S. Pat. No. 4,082,906 avoids noble metal electrodes by using ceramic dielectric compositions including LiF and rapid low temperature sintering (.ltoreq.900.degree. C.) in air. Due to the low sintering temperature of these materials it is possible to form a multilayer ceramic capacitor, hereinafter MLC, with up to 100 percent silver internal electrodes. The manufacturing cost of such MLCs is significantly reduced because they do not require electrodes containing large amounts of noble metals, such as Pd, Pt, or Au. However this process cannot be used with easily oxidized base metals such as copper.
Alternative approaches for producing ceramic capacitors with inexpensive metal electrodes which employ ceramic dielectric compositions produced by high temperature sintering (.gtoreq.1300.degree. C.) of barium titanate in reducing or non-oxidizing atmospheres with selected modifiers, are disclosed in U.S. Pat. Nos. 3,920,781; 4,386,985; 4,089,813. The internal electrodes in these ceramic capacitors are a base metal, such as nickel or nickel alloy having a melting point greater than the sintering temperature. The non-oxidizing atmospheres are necessary to avoid oxidation of the base metal during sintering and concomitant loss of its electrical conductance.
It is also important in some of these processes to employ BaTiO.sub.3 having a (BaO:TiO.sub.2) mole ratio greater than 1 to avoid reduction of the metal oxides in the dielectric compositions to an electrically conductive state or otherwise deteriorate the dielectric's properties.
Yet another approach to avoiding the use of expensive noble metal electrodes in ceramic capacitors involves forming sintered ceramic bodies having void spaces therein and thereafter injecting lower melting point base metals, such as lead, into the void spaces to form the electrodes. Such processes are described in U.S. Pat. Nos. 3,679,950 and 3,879,645.
An object of the present invention is to provide dielectric compositions that may be sintered in non-oxidizing atmospheres at low temperatures, about 950.degree. C. or less, and useful with inexpensive base metal electrode material for forming MLCs.
Another object of the present invention is to provide a method for manufacturing ceramic capacitors at low sintering temperatures, i.e. less than about 950.degree. C., with base metal electrodes in non-oxidizing atmospheres wherein the oxygen partial pressure (PO.sub.2) is adjusted to avoid base metal oxidation and deterioration of the ceramics electrical properties.
Yet another object of this invention relates to ceramic dielectric compositions that can be sintered into dense ceramic bodies at ultra-low temperatures (.ltoreq.950.degree. C.) in atmospheres having reduced oxygen concentrations.
The dielectric compositions of this invention allow the manufacture of high quality MLCs with low cost base metal electrodes, such as copper, nickel, cobalt, iron and/or their alloys. The manufacturing costs are reduced as compared to the prior art because copper, nickel, cobalt, iron and their alloys are much less expensive than silver and nobel metals. Moreover, sintering at lower temperatures reduces the energy costs and increases the lifetime of processing equipment such as kilns.